SAT I now known as the SAT reasoning test measures your critical thinking abilities through Critical reading, Mathematics and Writing. Each section is mandatory to take and has a score out of 800. If you're talking about how many times to take the SAT reasoning test, then I'd suggest once (with intensive preparation)
Most universities in the U.S. require this test as a component of your application
Now, as for the SAT II now known as SAT Subject test is subject specific and includes 5 categories: English, History, Mathematics, Science and Languages (R only or R/L). Most U.S. universities require 2 Subject tests (with the exception of Harvard, which requires 3). I recommend taking 3 subject tests in your area of competence and interest and then send the best two to them. For example: If you're competent in French and love Biology and want to study Mathematics, I'd recommend to take those three if you can excel in them. It wouldn't look nice for example if you take Physics (which you don't like) and end up with a very low score. The choice is up to you.
Hopefully this description helps. Feel free to ask any question you have.
